Conventionally, optical glass having a high refractive index, and with a high dispersion region are typically of a composition containing a large amount of lead oxide. Because of favorable glass stability and a low glass transition point (Tg), they have been used for precision mold press molding. For example, Patent Document 1 discloses an optical glass for a precision mold press which contains a large amount of lead oxide.
However, a reduction condition is maintained in order to prevent oxidation of the mold when molding by a precision mold press is performed. Therefore, when the glass component contains lead oxide, lead reduced from the glass surface break off and adheres to the mold surface. Accordingly, there have been problems in that the precision face of the mold is not maintained. Also, lead oxide is toxic to the environment so that glass free from lead oxide has been desired.
To meet such needs, many types of optical glass for press molding, having a high refractive index, with a high dispersion region, and not containing lead oxide, have been developed. However, most of those are of a phosphate-based system. For example, Patent Document 2 and Patent Document 3 disclose P2O5—Nb2O5—WO3—(K2O, Na2O, Li2O) based glass, and Patent Document 4 discloses P2O5—Nb2O5—TiO2—Bi2O3—Na2O based glass. However, many of these types of glass have a Tg of higher than 480° C., although such Tgs may be referred to as being low. Furthermore, a large amount of Nb2O5 must be introduced for achieving a high refractive index and high dispersion; therefore, there may be disadvantages in that devitrification resistance of the glass is not so high. Moreover, an optical glass having optical constants such that its refractive index is 1.95 or greater, and its dispersion is 20 or less has not been obtained from the phosphoric acid-based glass.
Furthermore, compositions including a large amount of Bi2O3 are well known for glasses with low Tg. For example, Nonpatent Documents 1, 2, 3, and 4 disclose Bi2O3—Ga2O3—PbO based glass, Bi2O3—Ga2O3-(Li2O, K2O, Cs2O) based glass, and Bi2O3—GeO2 based glass. Although these types of glass have a Tg of 480° C. or lower, an absorption edge of the glass is more than 450 nm. Hence, transparency in the visible region may be drastically lost, and thus, these types of glass cannot be used as an optical lens required to have high transparency in the visible region.    [Patent Document 1] Japanese Patent Application Laid Open No. H1-308843    [Patent Document 2] Japanese Patent Application Laid Open No. 2003-321245    [Patent Document 3] Japanese Patent Application Laid Open No. H8-157231    [Patent Document 4] Japanese Patent Application Laid Open No. 2003-300751    [Nonpatent Document 1] Physics and Chemistry of Glasses, p 119, Vol. 27, No. 3, June 1986    [Nonpatent Document 2] Glass Technology, p 106, Vol. 28, No. 2, April 1987    [Nonpatent Document 3] American Ceramic Society Bulletin, p 1543, Vol. 71, No. 10, October 1992    [Nonpatent Document 4] American Ceramic Society, p 1017, Vol. 77, No. 4, October 1994